It has long been known that techniques of spatial multiplexing can be used to improve the spectral efficiency of wireless networks. (Spectral efficiency describes the transmitted data rate per unit of frequency, typically in bits per second per Hz.) In typical examples of spatial multiplexing, a multiple array of transmit antennas sends a superposition of messages to a multiple array of receive antennas. The channel state information (CSI), i.e. the channel coefficients between the respective transmit-receive antenna pairs, is assumed known. Provided that there is low correlation among the respective channel coefficients, the CSI can be used by the transmitter, or the receiver, or both, to define a quasi-independent channel for each of the transmitted messages. As a consequence, the individual messages are recoverable at the receiving antenna array.
More recently, experts have proposed extensions of the spatial multiplexing technique, in which a multiplicity of mobile or stationary user terminals (referred to herein as “access terminals” or “ATs”) are served simultaneously in the same time-frequency slots by an even larger number of base station antennas or the like, which we refer to herein as “service antennas”. Particularly when the number of service antennas is much greater than the number of ATs, such networks may be referred to as “Large-Scale Antenna Systems (LSAS)”. Theoretical studies predict that the performance of LSAS networks scales favorably with increasing numbers of service antennas. In particular, there are gains not only in the spectral efficiency, but also in the energy efficiency. (The energy efficiency describes the ratio of total data throughput to total transmitted power, and is measured, e.g., in bits per Joule.)
Although the theoretical studies are favorable, practical difficulties still stand in the way of a large, geographically extensive LSAS. One problem, among others, is how to make tractable the collection and processing of CSI gathered from many antennas, which may number in the hundreds or more, spread over a geographical area that may cover hundreds of square kilometers or more.